Electrical circuits that attenuate electrical signals are known as attenuators. Such attenuators are commonly used to control the gain of radio frequency amplifier stages. For example, a high power amplifier typically consists of a plurality of lower power amplifier stages arranged in a parallel configuration. The output of each amplifier stage is combined with the other amplifier stage outputs in a combiner to produce a desired output power. To minimize combining losses in the combiner, each amplifier stage needs to provide approximately equal gain. Thus, variable attenuators are often positioned at the input to each amplifier stage to control the stage's gain in light of inevitable component tolerances. In addition, the variable attenuators typically maintain constant input and output impedances (e.g., 50 ohms) to minimize their loading effect on the amplifier stages' input impedances.
One such prior art constant impedance attenuator 100 is schematically depicted in FIG. 1. This attenuator 100 is commonly known as a bridged-T attenuator. The bridged-T attenuator 100 has an input terminal 101, an output terminal 102, and a signal common 103 and attenuates an electrical signal incident at the input terminal 101. The attenuator 100 comprises four resistors 105-108. Resistor 105 is connected between the input terminal 101 and node 110, while resistor 106 is connected between node 110 and the output terminal 102. Resistor 107 is connected between the input terminal 101 and the output terminal 102, while resistor 108 is connected between node 110 and the signal common 103. Resistors 105, 106 have equal resistances, while resistors 107, 108 have inversely proportional resistances to maintain the attenuator's constant input and output impedances. Resistors 107, 108 are typically variable resistors that provide attenuation adjustment to the attenuator 100.
To adjust the attenuation of the attenuator 100 to a particular level while maintaining the attenuator's constant input and output impedances, the resistance of resistor 108 must be varied inversely with respect to the resistance of resistor 107. Therefore, if the resistance of resistor 107 is increased to a desired level, the resistance of resistor 108 must be correspondingly decreased to an inversely proportional level, and vice versa. Thus, at least one of the variable resistors 107, 108 of the bridged-T attenuator 100 must be capable of having its resistance decreased in order to maintain the constant input and output impedances.
Amplifier stages in high power amplifiers are typically fabricated on alumina ceramic substrates to facilitate heat transfer of heat generated by the amplifier stages to a heat sink. When an amplifier stage is constructed on ceramic, the resistors used in the amplifier stages are commonly fabricated as thick film resistors deposited on the substrate using well-known thick film fabrication techniques. In addition, an amplifier stage necessitating a variable attenuator typically incorporates the variable attenuator on the ceramic substrate containing the amplifier stage to permit adjustment, or tuning, of the attenuator's variable resistors during the tuning of any variable elements of the amplifier stage (e.g., distributed capacitors and resistors). However, thick film resistors can only have their resistances increased during tuning (e.g., with a laser). Thus, the use of thick film resistors in a bridged-T attenuator 100 is restricted due to the requirement that the branch resistance between node 110 and the signal common 103 decrease when the resistance of resistor 107 increases.
Therefore, a need exists for an electrical circuit that facilitates the use of thick film resistors as the variable elements of a constant impedance variable attenuator.